WambuCombu21 :
Jester Maroc :
For that system you will find more benefits from 8gb than 1TB HDD storage.
Go for a small cheap HDD, and 8gb RAM. You can add an SSD later to help with performance and if you need space you can add a 2-4TB drive later as well.
What's the best ammount for an hhd under 1TB
I am assuming you mean "best amount of storage"? I find a 120gb SSD to be sufficient for a gaming PC unless you have a huge library and want all your games installed locally.
Since you are on a tight budget I assume you do not yet have a very large library so anything 120gb and up is enough to allow for a Win 10 installation, a few programs, some music, and 2-3 (medium sized modern) games. It depends on the need you have for storage.